At present, a tubulin-binding agent, which generally exerts anticancer effects by depolymerizing or stabilizing microtubule, is one of the most effective anticancer drugs in clinical applications. Microtubule is an important component of cells, and constitutes mitotic apparatus. It involves cell movement, attachment and intracellular transport. Vinca alkaloids drugs, especially vincristine and vinblastine, have been clinically used for many years. In recent years, it is shown that vinorelbine can be used in the treatment of breast cancer, and the study of vinflunine has entered into the clinical trial stage. This class of drugs belongs to an anti-mitotic agent, and can inhibit assembly in mitosis. Besides, another class of clinical active drugs, such as paclitaxel, exerts its anticancer effect by promoting the formation of non-functional microtubules. Thus, the same anticancer effect can be achieved by interference with the tubulin using the drugs with totally different action mechanisms. Although these existing drugs are clinically active, they have poor efficacy on patients with advanced cancers, and may harm normal tissues. In recent years, a type of natural polyhydric stilbene compounds, combretastatin, which was isolated from the bark of African bush willow, has caused a lot of attention, among which the study of combretastatin A4 further clinically developed by Oxigene Company has made a promising progress.
Combretastatin A4 is the compound having the highest activity and the simplest structure in the combretastatin family. Studies have demonstrated that its possible mechanism of anticancer is binding rapidly with tubulin to realize the antimitotic function. Additionally, there are studies showing that this compound can target and destroy tumor cells without damaging blood vessels in normal tissues.
Combretastatin A4 has a basic structure of two benzene rings connected by an ethylene bridge, in which the one with three methoxy substituents is called ring A, and the other is called ring B. Its chemical name is cis-1-(3,4,5-trimethoxy phenyl)-2-(3′-hydroxy-4′-methoxy phenyl)ethylene. It can compete with colchicine for binding sites on tubulin. The currently available 1,2-stilbene derivatives are described in the existing publications, such as U.S. Pat. Nos. 4,996,237, 5,561,122 and 5,674,906, and the derivatives with 3,4,5-trimethoxyphenyl structure are described in De Martino, Gabriella et. al., J. Med. Chem. 2004, 47(25), 6120-6123.
Although combretastatin A4 has the cytotoxic activity and anti-tubulin polymerization activity in vitro, the cis-stilbene is structurally unstable and readily isomerizes into the trans-configuration. The cis-configuration, however, has stronger anti-tubulin activity than the trans-configuration. The structure-activity relationship indicates that the prerequisite for the activity is that the two benzene rings in the structure of combretastatin A4 take the cis-configuration. Accordingly, a series of analogues are obtained by replacing the ethenyl group with a rigid ring, such as a 4, 5 or 6-membered ring or a fused heterocyclic ring, so as to avoid the problem of stilbene isomerization and, in the meantime, retain substantially the cytotoxic activity and anti-tubulin polymerization activity of combretastatin A4. In addition, the A ring in the structure of combretastatin A4, i.e., the 3,4,5-trimethoxyphenyl group, is also necessary to maintain the high cytotoxic activity.